The present invention relates to light emitting diode (LED) light systems for channel letters in signs, particularly to LED systems serially connected with shunts for each LED, and circuits for driving the LEDs which provide circuit safety protection, and to provide an LED module for series connection with other LED modules.
LEDs have become very popular for providing illumination in the sign industry due to their superior efficiency compared to incandescent and fluorescent bulbs, and the avoidance of harmful mercury in fluorescent bulbs. LEDs are also smaller and generally easier to ship, store and install than the prior light sources. Advancements in LED technology have also made LEDs even more desirable.
LEDs have been used in parallel circuits in the sign industry, but a disadvantage is that increased current is required to drive them in parallel. Such systems typically use a low voltage, high current class 2 power supply.
In the parallel LED connection arrangement, the low voltage power supply must have wires to carry the total current of all LEDs operating from one power supply. The total current could be as high as 5 amps, requiring heavy gauge wire of higher cross-section which adds to cost. Also, the power losses due to IR drops in the wires become significant. The use of higher total currents also presents a safety issue.
The conventional parallel LED system using a constant low voltage power supply requires that some ballasting means be provided to limit the current into the parallel connected LEDs to prevent excessive change in operating current with temperature variations and as a result of manufacturing tolerance. These ballasting means all dissipate additional power to some degree over and above that required for operation of the LEDs. The provision of a ballasting means is an additional cost in the manufacturing, and also results in additional cost in operation due to the waste of power. Also, the ballasting means can also fail during operation.
While it is possible to connect the LEDs in series, rather than in parallel, to avoid many of the problems of parallel connection discussed above, the single path series arrangement has a disadvantage in that a failure of one LED in the series would cause an open circuit in the single path, with the result that none of the still operative, non-failed LEDs would continue to be lit. Because none of the operative LEDs would be illuminated, it is difficult and time consuming to identify the failed LED to replace it. Also, a failure of even one LED would result in downtime of the entire circuit, which may come at an in opportune time. The entire system would be inoperative until troubleshooting is performed by service personnel, which may be costly especially if the repair needs to be done immediately, or after business hours. Once the failed LED is identified, replacement typically requires cutting the wire and hard-wiring in the replacement LED.